In recent years, demand for high-capacity non volatile memory devices for storage of data of music, pictures, and other information has been increasing with progress of digital technology for electric appliances. A notable means to meet this demand is a resistance random access memory device (hereinafter referred to as a ReRAM). A ReRAM includes memory cells which are nonvolatile memory elements each having resistance changeable according to an electrical signal applied thereto and thereafter retained. This notability is due to characteristics of the nonvolatile memory element, such as its structure which is so simple that density can be easily increased, and which is compatible with conventional semiconductor processes.
Such nonvolatile memory elements are categorized into two major types according to materials for variable resistance layers (variable resistance material). A variable resistance nonvolatile memory element of one type includes a perovskite-based material disclosed in, for example, Patent Literature (PTL) 1, such as Pr1-xCaxMnO3 (PCMO), La1-xSrxMnO3 (LSMO), or GdBaCoxOy (GBCO), as a variable resistance material.
A variable resistance nonvolatile memory element of the other type includes a binary transition metal oxide as a variable resistance material. Compared to the perovskite-based material, composition and structure of binary transition metal oxides are so simple that composition control and film formation in manufacturing can be easily performed. Furthermore, binary transition metal oxides advantageously have rather favorable compatibility with semiconductor processes, and thus there has been considerable research on binary transition metal oxides.
Although much about the mechanism of resistance change still remains unknown, recent researches have found that a likely cause of such resistance change is change in defect density of conductive filaments formed in a binary transition metal oxide by an oxidation-reduction reaction (for example, see PTL 2 and Non Patent Literature (NPL) 1).
FIG. 17 is a cross-sectional view showing a configuration of a conventional nonvolatile memory element 1400 disclosed in PTL 2.
The nonvolatile memory element 1400 initially has a structure in which a variable resistance layer 1405 including a transition metal oxide is located between a first electrode 1403 and a second electrode 1406 ((a) in FIG. 17). Then, a voltage (initial breakdown voltage) is applied between the first electrode 1403 and the second electrode 1406, so that a filament 1405c is formed which serves as a current path between the first electrode 1403 and the second electrode 1406 (that is, a portion where a current flowing between the first electrode 1403 and the second electrode 1406 locally has a higher density) ((b) in FIG. 17).